You will need to be a Super Administrator or a User Administrator to do this.
If you have end-dated a colleague and you need to re-activate them, you can do this in your organisation's user settings.
Go to your organisation's page by clicking on the building icon or your organisation's name in the top right of the menu. On the left menu in your organisation page, click organisation settings > user settings > users.
Find the inactive user by clicking on the search icon, checking show deactivated users and clicking search.
Click on the users name, remove the end date value and click save changes.
The user should now have access to your organisation.